System and method to prevent driveline float in lash region

ABSTRACT

A vehicle includes an engine and an electric machine coupled to a gearbox through a torque converter. The vehicle includes a controller programmed to command an engine torque and an electric machine torque to achieve a predetermined positive torque at the input of the torque converter when a driver demand torque at the torque converter input decreases to fall within a range between the predetermined positive torque and a predetermined negative torque.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 15/345,705filed Nov. 8, 2016, the disclosure of which is hereby incorporated inits entirety by reference herein.

TECHNICAL FIELD

This application generally relates to a torque control strategy for ahybrid vehicle

BACKGROUND

Vehicles include a transmission for transmitting power and torque to thedrive wheels of the vehicle. Transmissions are available in a variety ofconfigurations including manual, automatic, and hybrid. Vehicletransmissions may be comprised of a number of gears that mesh togetherto transfer torque through the transmission. Gears must be designed withsome spacing such that the gears can easily rotate when meshed withother gears. As the gears wear during usage, the spacing may change.Because of the spacing, there may be lash or play in the gears. Lashoccurs when the gears are not in full contact with one another as canhappen when the torque changes direction. Undesirable noise may occurwhen the gears come in contact with one another.

SUMMARY

A vehicle includes an engine and an electric machine coupled to agearbox through a torque converter. The vehicle further includes acontroller programmed to, in response to a driver demand torque at aninput of the torque converter decreasing to fall within a range betweena predetermined positive torque and a predetermined negative torque,command and engine torque and an electric machine torque to achieve thepredetermined positive torque at the input of the torque converter.

A powertrain control system includes an engine and an electric machinecoupled to a gearbox through a torque converter. The powertrain controlsystem further includes a controller programmed to, in response to adriver demand torque at the torque converter being between an impellerstall torque of the torque converter and a predetermined negative torqueand a torque at the torque converter being greater than the impellerstall torque, command the engine and the electric machine to cause thetorque to be the impeller stall torque.

A method includes operating, by a controller, an engine and an electricmachine that are coupled to a gearbox through a torque converter tocause a torque of the torque converter to achieve an impeller stalltorque in response to a driver demand torque at the torque converterbeing between the impeller stall torque and a predetermined negativetorque and the torque being greater than the impeller stall torque.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a vehicle with a hybrid powertrain.

FIG. 2 is a plot of a possible trajectory of a driver demand torque atan input to a torque converter that is transitioning from positive tonegative.

FIG. 3 is a plot of a possible trajectory of a driver demand torque atan input to a torque converter that is transitioning from negative topositive.

FIG. 4 is a flowchart of a possible sequence of operations for a vehiclepowertrain system.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the embodiments. Asthose of ordinary skill in the art will understand, various featuresillustrated and described with reference to any one of the figures canbe combined with features illustrated in one or more other figures toproduce embodiments that are not explicitly illustrated or described.The combinations of features illustrated provide representativeembodiments for typical applications. Various combinations andmodifications of the features consistent with the teachings of thisdisclosure, however, could be desired for particular applications orimplementations.

Referring to FIG. 1, a schematic diagram of a hybrid electric vehicle(HEV) 110 is illustrated according to an embodiment of the presentdisclosure. FIG. 1 illustrates representative relationships among thecomponents. Physical placement and orientation of the components withinthe vehicle may vary. The HEV 110 includes a powertrain 112. Thepowertrain 112 includes an engine 114 that drives a transmission 116,which may be referred to as a modular hybrid transmission (MHT). As willbe described in further detail below, transmission 116 includes anelectric machine 118 such as an electric motor/generator (M/G), anassociated traction battery 120, a torque converter 122, and a multiplestep-ratio automatic transmission or gearbox 124. The electric machine118 may also be reference to as the M/G (motor/generator).

The engine 114 and the M/G 118 are both drive sources for the HEV 110.The engine 114 generally represents a power source that may include aninternal combustion engine such as a gasoline, diesel, or natural gaspowered engine, or a fuel cell. The engine 114 generates an engine powerand corresponding engine torque that is supplied to the M/G 118 when adisconnect clutch 126 between the engine 114 and the M/G 118 is at leastpartially engaged. A controller associated with the engine 114 may beconfigured to control engine power and engine torque to correspondingcommanded values. The M/G 118 may be implemented by any one of aplurality of types of electric machines. For example, M/G 118 may be apermanent magnet synchronous motor. Power electronics 156 conditiondirect current (DC) power provided by the traction battery 120 to therequirements of the M/G 118, as will be described below. For example,power electronics 156 may provide three phase alternating current (AC)to the M/G 118.

When the disconnect clutch 126 is at least partially engaged, power flowfrom the engine 114 to the M/G 118 or from the M/G 118 to the engine 114is possible. For example, the disconnect clutch 126 may be engaged andM/G 118 may operate as a generator to convert rotational energy providedby a crankshaft 128 and M/G shaft 130 into electrical energy to bestored in the traction battery 120. The disconnect clutch 126 can alsobe disengaged to isolate the engine 114 from the remainder of thepowertrain 112 such that the M/G 118 can act as the sole drive sourcefor the HEV 110. Shaft 130 extends through the M/G 118. The M/G 118 iscontinuously drivably connected to the shaft 130, whereas the engine 114is drivably connected to the shaft 130 only when the disconnect clutch126 is at least partially engaged.

The M/G 118 is connected to the torque converter 122 via shaft 130. Thetorque converter 122 is therefore connected to the engine 114 when thedisconnect clutch 126 is at least partially engaged. The torqueconverter 122 includes an impeller fixed to M/G shaft 130 and a turbinefixed to a transmission input shaft 132. The torque converter 122 thusprovides a hydraulic coupling between shaft 130 and transmission inputshaft 132. The torque converter 122 transmits power from the impeller tothe turbine when the impeller rotates faster than the turbine. Themagnitude of the turbine torque and impeller torque generally dependupon the relative speeds. When the ratio of impeller speed to turbinespeed is sufficiently high, the turbine torque is a multiple of theimpeller torque. A torque converter bypass clutch 134 may also beprovided that, when engaged, frictionally or mechanically couples theimpeller and the turbine of the torque converter 122, permitting moreefficient power transfer. The torque converter bypass clutch 134 may beoperated as a launch clutch to provide smooth vehicle launch.Alternatively, or in combination, a launch clutch similar to disconnectclutch 126 may be provided between the M/G 118 and gearbox 124 forapplications that do not include a torque converter 122 or a torqueconverter bypass clutch 134. In some applications, disconnect clutch 126is generally referred to as an upstream clutch and launch clutch 134(which may be a torque converter bypass clutch) is generally referred toas a downstream clutch.

The gearbox 124 may include gear sets (not shown) that are selectivelyplaced in different gear ratios by selective engagement of frictionelements such as clutches and brakes (not shown) to establish thedesired multiple discrete or step drive ratios. The gearbox 124 mayprovide a predetermined number of gear ratios that may range from a lowgear (e.g., first gear) to a highest gear (e.g., Nth gear). An upshiftof the gearbox 124 is a transition to a higher gear. A downshift of thegearbox 124 is a transition to a lower gear. The friction elements arecontrollable through a shift schedule that connects and disconnectscertain elements of the gear sets to control the ratio between atransmission output shaft 136 and the transmission input shaft 132. Thegearbox 124 is automatically shifted from one ratio to another based onvarious vehicle and ambient operating conditions by an associatedcontroller, such as a powertrain control unit (PCU) 150. The gearbox 124then provides powertrain output torque to output shaft 136.

It should be understood that the hydraulically controlled gearbox 124used with a torque converter 122 is but one example of a gearbox ortransmission arrangement; any multiple ratio gearbox that accepts inputtorque(s) from an engine and/or a motor and then provides torque to anoutput shaft at the different ratios is acceptable for use withembodiments of the present disclosure. For example, gearbox 124 may beimplemented by an automated mechanical (or manual) transmission (AMT)that includes one or more servo motors to translate/rotate shift forksalong a shift rail to select a desired gear ratio. As generallyunderstood by those of ordinary skill in the art, an AMT may be used inapplications with higher torque requirements, for example.

As shown in the representative embodiment of FIG. 1, the output shaft136 is connected to a differential 140. The differential 140 drives apair of wheels 142 via respective axles 144 connected to thedifferential 140. The differential 140 may include a predetermined gearratio between the output shaft 136 and the axles 144. The differential140 transmits approximately equal torque to each wheel 142 whilepermitting slight speed differences such as when the vehicle turns acorner. Different types of differentials or similar devices may be usedto distribute torque from the powertrain to one or more wheels. In someapplications, torque distribution may vary depending on the particularoperating mode or condition, for example.

The powertrain 112 further includes an associated powertrain controlunit (PCU) 150. While illustrated as one controller, the PCU 150 may bepart of a larger control system and may be controlled by various othercontrollers throughout the vehicle 110, such as a vehicle systemcontroller (VSC). It should therefore be understood that the powertraincontrol unit 150 and one or more other controllers can collectively bereferred to as a “controller” that controls various actuators inresponse to signals from various sensors to control functions such asstarting/stopping engine 114, operating M/G 118 to provide wheel torqueor charge traction battery 120, select or schedule transmission shifts,etc. Controller 150 may include a microprocessor or central processingunit (CPU) in communication with various types of computer readablestorage devices or media. Computer readable storage devices or media mayinclude volatile and nonvolatile storage in read-only memory (ROM),random-access memory (RAM), and keep-alive memory (KAM), for example.KAM is a persistent or non-volatile memory that may be used to storevarious operating variables while the CPU is powered down.Computer-readable storage devices or media may be implemented using anyof a number of known memory devices such as PROMs (programmableread-only memory), EPROMs (electrically PROM), EEPROMs (electricallyerasable PROM), flash memory, or any other electric, magnetic, optical,or combination memory devices capable of storing data, some of whichrepresent executable instructions, used by the controller in controllingthe engine or vehicle.

The controller 150 communicates with various engine/vehicle sensors andactuators via an input/output (I/O) interface that may be implemented asa single integrated interface that provides various raw data or signalconditioning, processing, and/or conversion, short-circuit protection,and the like. Alternatively, one or more dedicated hardware or firmwarechips may be used to condition and process particular signals beforebeing supplied to the CPU. As generally illustrated in therepresentative embodiment of FIG. 1, PCU 150 may communicate signals toand/or from engine 114, disconnect clutch 126, M/G 118, launch clutch134, transmission gearbox 124, and power electronics 156. Although notexplicitly illustrated, those of ordinary skill in the art willrecognize various functions or components that may be controlled by PCU150 within each of the subsystems identified above. Representativeexamples of parameters, systems, and/or components that may be directlyor indirectly actuated using control logic executed by the controllerinclude fuel injection timing, rate, and duration, throttle valveposition, spark plug ignition timing (for spark-ignition engines),intake/exhaust valve timing and duration, front-end accessory drive(FEAD) components such as an alternator, air conditioning compressor,battery charging, regenerative braking, M/G operation, clutch pressuresfor disconnect clutch 126, launch clutch 134, and transmission gearbox124, and the like. Sensors communicating input through the I/O interfacemay be used to indicate turbocharger boost pressure, crankshaft position(PIP), engine rotational speed (RPM), wheel speeds (WS1, WS2), vehiclespeed (VSS), coolant temperature (ECT), intake manifold pressure (MAP),accelerator pedal position (PPS), ignition switch position (IGN),throttle valve position (TP), air temperature (TMP), exhaust gas oxygen(EGO) or other exhaust gas component concentration or presence, intakeair flow (MAF), transmission gear, ratio, or mode, transmission oiltemperature (TOT), transmission turbine speed (TS), torque converterbypass clutch 134 status (TCC), deceleration or shift mode (MDE), forexample.

Control logic or functions performed by PCU 150 may be represented byflow charts or similar diagrams in one or more figures. These figuresprovide representative control strategies and/or logic that may beimplemented using one or more processing strategies such asevent-driven, interrupt-driven, multi-tasking, multi-threading, and thelike. As such, various steps or functions illustrated may be performedin the sequence illustrated, in parallel, or in some cases omitted.Although not always explicitly illustrated, one of ordinary skill in theart will recognize that one or more of the illustrated steps orfunctions may be repeatedly performed depending upon the particularprocessing strategy being used. Similarly, the order of processing isnot necessarily required to achieve the features and advantagesdescribed herein, but is provided for ease of illustration anddescription. The control logic may be implemented primarily in softwareexecuted by a microprocessor-based vehicle, engine, and/or powertraincontroller, such as PCU 150. Of course, the control logic may beimplemented in software, hardware, or a combination of software andhardware in one or more controllers depending upon the particularapplication. When implemented in software, the control logic may beprovided in one or more computer-readable storage devices or mediahaving stored data representing code or instructions executed by acomputer to control the vehicle or its subsystems. The computer-readablestorage devices or media may include one or more of a number of knownphysical devices which utilize electric, magnetic, and/or opticalstorage to keep executable instructions and associated calibrationinformation, operating variables, and the like.

An accelerator pedal 152 is used by the driver of the vehicle to providea demanded torque, power, or drive command to propel the vehicle 110. Ingeneral, depressing and releasing the accelerator pedal 152 generates anaccelerator pedal position signal that may be interpreted by thecontroller 150 as a demand for increased power or decreased power,respectively. Based at least upon input from the pedal, the controller150 commands torque from the engine 114 and/or the M/G 118. Theaccelerator pedal position signal may be used to generator a drivertorque demand that represents an amount of torque to be applied at thedrive wheels 142.

The controller 150 may also control the timing of gear shifts within thegearbox 124, as well as engagement or disengagement of the disconnectclutch 126 and the torque converter bypass clutch 134. Like thedisconnect clutch 126, the torque converter bypass clutch 134 can bemodulated across a range between the engaged and disengaged positions.This produces a variable slip in the torque converter 122 in addition tothe variable slip produced by the hydrodynamic coupling between theimpeller and the turbine. Alternatively, the torque converter bypassclutch 134 may be operated as locked or open without using a modulatedoperating mode depending on the particular application.

To drive the vehicle 110 with the engine 114, the disconnect clutch 126is at least partially engaged to transfer at least a portion of theengine torque through the disconnect clutch 126 to the M/G 118, and thenfrom the M/G 118 through the torque converter 122 and gearbox 124. TheM/G 118 may assist the engine 114 by providing additional power to turnthe shaft 130. This operation mode may be referred to as a “hybrid mode”or an “electric assist mode.”

To drive the vehicle 110 with the M/G 118 as the sole power source, thepower flow remains the same except the disconnect clutch 126 isolatesthe engine 114 from the remainder of the powertrain 112. Combustion inthe engine 114 may be disabled or otherwise OFF during this time toconserve fuel. The traction battery 120 transmits stored electricalenergy through a high-voltage (HV) bus 154 to power electronics 156 thatmay include an inverter, for example. The high-voltage bus 154 includeswiring and conductors for conducting current between modules and mayinclude a positive-side conductor and a negative- or return-sideconductor. The power electronics 156 convert DC voltage from thetraction battery 120 into AC voltage to be used by the M/G 118. The PCU150 commands the power electronics 156 to convert voltage from thetraction battery 120 to an AC voltage provided to the M/G 118 to providepositive or negative torque to the shaft 130. This operation mode may bereferred to as an “electric only” operation mode.

In any mode of operation, the M/G 118 may act as a motor and provide adriving force for the powertrain 112. Alternatively, the M/G 118 may actas a generator and convert kinetic energy from the powertrain 112 intoelectric energy to be stored in the traction battery 120. The M/G 118may act as a generator while the engine 114 is providing propulsionpower for the vehicle 110, for example. The M/G 118 may additionally actas a generator during times of regenerative braking in which rotationalenergy from spinning wheels 142 is transferred back through the gearbox124 and is converted into electrical energy for storage in the tractionbattery 120.

A brake pedal 170 is used by the driver of the vehicle to provide abrake demand signal for braking or negative torque to slow the vehicle.In general, depressing and releasing the brake pedal 170 generates abrake pedal position signal that may be interpreted by the controller150 as a demand for increased braking or decreased braking,respectively. Based at least upon input from the brake pedal 170, thecontroller 150 commands braking torque from vehicle brakes (notillustrated). The vehicle brakes generally include friction brakes. TheM/G 118 may additionally act as a generator to provide regenerativebraking, in which rotational energy from spinning wheels 142 istransferred back through the gearbox 124 and is converted intoelectrical energy for storage in the battery 120.

It should be understood that the schematic illustrated in FIG. 1 ismerely exemplary and is not intended to be limiting. Otherconfigurations are contemplated that utilize selective engagement ofboth an engine and a motor to transmit torque through the transmission.For example, the M/G 118 may be offset from the crankshaft 128, anadditional motor may be provided to start the engine 114, and/or the M/G118 may be provided between the torque converter 122 and the gearbox124. Other configurations are contemplated without deviating from thescope of the present disclosure.

The vehicle 110 may utilize the M/G 118 to start the engine 114. The PCU150 may command the disconnect clutch 126 to close and request torquefrom the M/G 118 via the power electronics 156. The torque from the M/G118 rotates the engine 114 so that the engine speed increases above apredetermined speed at which time the engine 114 may be commanded toprovide fuel and spark to maintain continued engine rotation. The torqueconverter 122 may provide some torsional isolation during enginecranking and initial startup. In some vehicle configurations, alow-voltage starter motor 168 may also be coupled to the engine 114 toprovide a secondary or backup means of starting the engine 114.

The vehicle 110 may further include a power converter module 158 and anauxiliary battery 160. The auxiliary battery 160 may be a low-voltagebattery such as a 12 Volt battery that is commonly used in automobiles.Terminals of the auxiliary battery 160 may be electrically coupled to alow-voltage bus 166. The low-voltage bus 166 includes wiring andconductors for conducting current between connected modules. The powerconverter 158 may be electrically coupled between the high-voltage bus154 and the low-voltage bus 166. The power converter module 158 may be aDC/DC converter that is configured to convert voltage from thehigh-voltage bus 154 to a voltage level compatible with the low-voltagebus 166. The power converter 158 may be further configured to convertvoltage from the low-voltage bus 166 to voltage compatible with thehigh-voltage bus 154. For example, the power converter 158 may beconfigured to provide a two-way flow of current between the high-voltagebus 154 and the low-voltage bus 166.

The gearbox 124 may be comprised of gears configured to mesh with oneanother to transfer torque from the transmission input shaft 132 to thetransmission output shaft 136. The gears are subject to lash which maybe caused by gaps between the gears. Driveline lash can be particularlynoticeable when the torque applied to the gear changes direction. Whentorque is applied in one direction for a period of time, the gears arein contact with one another. When the torque is reduced or reversed,there may be a period of time in which the gears are not in contact withone another. During this period of time, the gears may be floatingrelative to one another. At some time, the gears may come in contactagain. Noise, referred to as clunk, can be caused when the gears come incontact again.

During vehicle operation, a driver demand torque may be derived from theaccelerator pedal position. For example, a propulsive torque demand maybe a function of the accelerator pedal position and increases as theaccelerator pedal position increases (e.g., accelerator pedal isdepressed). The driver demand torque may be used to generate apowertrain torque command that may include torque commands to the engine114 and the M/G 118. Under normal driving conditions, the powertraintorque command may be a propulsive torque command or a braking torquecommand. In some situations, the powertrain torque command may becomprised of an accelerator pedal component and a brake pedal component.For example, a driver may be depressing both the accelerator pedal 152and the brake pedal 170 at the same time (e.g., two-pedal drivingstyle). In this situation, the powertrain torque command due to theaccelerator pedal 152 may be summed with powertrain torque command dueto the brake pedal 170. The net result could be a powertrain torquecommand near zero. Such a powertrain torque command may cause thedriveline to float near lash and create unacceptable driveline clunks.

For an open torque converter, there is a minimum input torque formaintain rotation of the torque converter 122. The torque below whichthe torque converter 122 no longer rotates may be referred to as thestall torque of the torque converter 122.

A strategy for preventing the requested powertrain torque from causingthe driveline to float near lash is described herein. A zone around thelash region (e.g., zero torque) may be defined and the powertrain torquecommand may be modified to prevent torque from being commanded in thelash region.

The driver demand torque may be a torque value that is desired to beapplied at the drive axle 144 for transfer to the wheels 142. The driverdemand torque may be satisfied by any combination of engine torque andelectric machine torque. Various strategies for determining the desiredtorque split between the engine and the electric machine are availableand are not discussed herein. Due to the various gear ratios andcomponent operation, the intermediate torques applied to the variouscomponents may be different. However, knowing the desired torque output,it is a simple matter to compute the torque at any point in thedriveline. For example, knowing the gearbox gear ratios in each gear,the torque at the output of the gearbox 124 can be computed. As such,torque values may be referenced at any point within the driveline withknowledge that, by proper scaling, the torque values may be reflected toany point in the driveline. For example, the driver demand torque may bedetermined at the input of the torque converter 122.

FIG. 2 depicts a graph of a possible trajectory of a driver demandtorque 200 at the input to the torque converter 122. Note that thedepicted driver demand torque 200 decreases over time and crosses zeroand become negative. For example, the driver may be in coast mode or hasapplied the brake pedal 170. Also defined on the graph are an uppertorque threshold 204 and a lower torque threshold 206. The upper torquethreshold 204 may be a predetermined positive torque value. The lowertorque threshold 206 may be a predetermined negative torque value.

The computation of the upper torque threshold 204 may be dynamic and thevalue may be adjusted periodically during vehicle operation. The uppertorque threshold 204 may be based on a model of the torque converter122. At low vehicle speeds, a minimum torque is required to maintainrotation of the impeller and the powertrain torque command may beprevented from falling below this minimum torque with an open torqueconverter. The impeller stall torque may be calculated as follows:

$\begin{matrix}{\tau_{imp}^{stall} = ( \frac{\omega_{imp}^{\min}}{K({SR})} )^{2}} & (1) \\{{SR} = \frac{\omega_{t}}{\omega_{imp}^{\min}}} & (2)\end{matrix}$where ω_(imp) ^(min) is the minimum impeller speed for stalling, ω_(t)is the present turbine speed, and K is the imp capacity factor curvewhich is a function of a speed ratio, SR.

Note that the speed ratio for this analysis is characterized by theturbine speed and the impeller stall speed. The speed ratio may becharacterized by testing data and represented by a curve or function.The stall torque may be a minimum value when the capacity factor (K) isnear a maximum value. The capacity factor may be near a maximum valuewhen the speed ratio is in a range about a value of one. That is, whenthe turbine speed is approximately equal to the impeller stall speed.

The upper torque threshold 204 may be defined by a maximum of theimpeller stall torque and a selectable calibration value. The selectablecalibration value may be useful when the impeller stall torque is nearzero. (e.g., when the turbine speed is near or greater than the impellerstall speed). The lower torque threshold 206 may be defined by apredetermined calibration value. The calibrated values may depend on thedriveline hardware and may be a function of the present gear of thegearbox 124.

Also depicted in FIG. 2 is the powertrain torque command 202 at theinput to the torque converter 122. The actual powertrain torque at theinput to the torque converter 122 may be a sum of the torque produced bythe engine 114 and the torque produced by the electric machine 118. Thepowertrain torque command 202 may be generated by any combination ofelectric machine torque and engine torque. A control strategy may beimplanted such that the powertrain torque command 202 does not permit arequested torque value between the upper torque threshold 204 and thelower torque threshold 206. The control strategy creates a “no-fly zone”about zero torque that includes a range of torque values that will notbe requested. When the driver demand torque 200 falls below the uppertorque threshold 204 and is greater than the lower torque threshold 206,the powertrain torque command 202 takes the value of the upper torquethreshold 204. The powertrain torque command 202 maintains the value ofthe upper torque threshold 204 until the driver demand torque 200increases above the upper torque threshold 204 or falls below the lowertorque threshold 206. When the driver demand torque 200 increases abovethe upper torque threshold 204 or falls below the lower torque threshold206, the powertrain torque command 202 takes on the value of the driverdemand torque 200 (e.g., powertrain torque command 202 equals the driverdemand torque 200).

FIG. 3 depicts a possible torque trajectory when the driver demandtorque 300 at the input to the torque converter 122 is a negative torquevalue. In this case, the driver demand torque 300 increases over timeand crosses zero to become positive. Also depicted are the upper torquethreshold 204 and the lower torque threshold 206. A powertrain torquecommand 302 is also depicted. The driver demand torque 300 increases toa value above the lower torque threshold 206. When the driver demandtorque 300 exceeds the lower torque threshold 206, the powertrain torquecommand 302 is set to the lower torque threshold 206. The powertraintorque command 302 retains the value of the lower torque threshold 206until the driver demand torque 300 rises above the upper torquethreshold 204. When the driver demand torque 300 is greater than orequal to the upper torque threshold 204, the powertrain torque command302 is set to the driver demand torque 300.

FIG. 4 depicts a flowchart of a possible sequence of operations forimplementing a powertrain control system according to the describedstrategies. The operations may be programmed in a controller (e.g., PCU150) that is part of the powertrain control system. At operation 400, adriver demand torque may be calculated. The driver demand torque may bea sum of an accelerator pedal torque component and a brake pedal torquecomponent. The driver demand torque may be an amount of torque desiredat the drive wheels. The driver demand torque may be computed at variouspoints in the powertrain. The driver demand torque at the input of thetorque converter may be computed based on the gear ratios of the gearbox124 and the differential 140. The driver demand torque at the input tothe torque converter may be based on a torque multiplication factor ofthe torque converter.

At operation 402, an impeller stall torque may be computed from a torqueconverter model. The impeller stall torque may be computed as describedabove. At operation 404, the upper torque threshold 204 may be computedas a maximum of the impeller stall torque and a calibration value. Atoperation 406, the lower torque threshold 206 may be computed. Forexample, the lower torque threshold 206 may be a predeterminedcalibration value.

At operation 408, the driver demand torque at the torque converter inputmay be compared to the upper torque threshold 204. If the driver demandtorque is greater than the upper torque threshold 204, operation 414 maybe executed. At operation 414, the driver demand torque may be requestedas the powertrain torque command. If the driver demand torque is lessthan or equal to the upper torque threshold 204, operation 410 may beperformed.

At operation 410, a check may be performed to determine if the driverdemand torque is between the upper torque threshold 204 and the lowertorque threshold 206. If the driver demand torque is not between theupper torque threshold 204 and the lower torque threshold 206, operation414 may be performed. In this case, the driver demand torque may be lessthan the lower torque threshold 206. At operation 414, the driver demandtorque may be requested as the powertrain torque command. If the driverdemand torque is between the upper torque threshold 204 and the lowertorque threshold 206, operation 412 may be performed.

At operation 412, the previous powertrain torque command is compared tothe upper torque threshold 204. If the previous powertrain torquecommand is greater than or equal to the upper torque threshold 204,operation 416 may be performed. At operation 416, the upper torquethreshold 204 may be requested as the powertrain torque request. If theprevious powertrain torque command is less than the upper torquethreshold 204, then operation 418 may be performed. At operation 418,the lower torque threshold 206 may be requested as the powertrain torquecommand.

Operation 420 may be performed after outputting the powertrain torquecommand. At operation 420, the control iteration may be ended. Theoperations may be periodically repeated to update the powertrain torquecommand over a drive cycle of the vehicle.

The powertrain torque command at the input of the torque converter 122may be achieved by operation of the engine 114 and electric machine 118.The electric machine 118 may be operated by commanding a torque outputof the electric machine 118. For example, the power electronics 156 mayreceive a commanded electric machine torque and control the torqueoutput of the electric machine 118 to achieve the commanded electricmachine torque. An engine control module or function may receive anengine torque command and may adjust engine operating parameters toachieve the engine torque command. The engine 114 may be operated toadjust engine torque by adjusting throttle position, valve timing,air/fuel mixture, spark/ignition timing and by starting/stopping theengine 114. In some modes of operation, the electric machine torque maybe applied along with an engine torque. In some cases, the electricmachine torque may modify the engine torque to achieve the powertraintorque command at the input of the torque converter 122. In other case,the engine 114 or the electric machine 118 may be operated alone tosatisfy the powertrain torque command. The combination of electricmachine torque and engine torque to satisfy the powertrain torquecommand may depend on the operating mode of the vehicle (e.g., enginerunning or engine stopped). In some cases, a combination of enginetorque and electric machine torque may be chosen to minimize fuelconsumption.

The control strategy described improves driver satisfaction by reducingdriveline clunk and powertrain noise. By monitoring the impeller stalltorque during the drive cycle, the operating limits are continuallyadjusted to ensure an adequate torque range for reducing clunk.

The processes, methods, or algorithms disclosed herein can bedeliverable to/implemented by a processing device, controller, orcomputer, which can include any existing programmable electronic controlunit or dedicated electronic control unit. Similarly, the processes,methods, or algorithms can be stored as data and instructions executableby a controller or computer in many forms including, but not limited to,information permanently stored on non-writable storage media such as ROMdevices and information alterably stored on writeable storage media suchas floppy disks, magnetic tapes, CDs, RAM devices, and other magneticand optical media. The processes, methods, or algorithms can also beimplemented in a software executable object. Alternatively, theprocesses, methods, or algorithms can be embodied in whole or in partusing suitable hardware components, such as Application SpecificIntegrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs),state machines, controllers or other hardware components or devices, ora combination of hardware, software and firmware components.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes mayinclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

What is claimed is:
 1. A vehicle comprising: an engine and an electricmachine coupled to a gearbox through a torque converter; and acontroller programmed to, in response to a driver demand torque at aninput of the torque converter decreasing to fall within a range betweena predetermined positive torque and a predetermined negative torque,command an engine torque and an electric machine torque to achieve thepredetermined positive torque at the input of the torque converter. 2.The vehicle of claim 1, wherein the controller is further programmed to,in response to the driver demand torque at the input decreasing belowthe predetermined negative torque, command the engine torque and theelectric machine torque to achieve the driver demand torque.
 3. Thevehicle of claim 1, wherein the controller is further programmed to, inresponse to the driver demand torque at the input increasing to fallwithin the range between the predetermined positive torque and thepredetermined negative torque, command the engine torque and theelectric machine torque to achieve the predetermined negative torque. 4.The vehicle of claim 1, wherein the controller is further programmed to,in response to the driver demand torque at the input increasing toexceed the predetermined positive torque, command the engine torque andthe electric machine torque to achieve the driver demand torque.
 5. Thevehicle of claim 1, wherein the predetermined positive torque is animpeller stall torque of the torque converter.
 6. The vehicle of claim5, wherein the controller is further programmed to estimate the impellerstall torque based on a model of the torque converter.
 7. The vehicle ofclaim 1, wherein the predetermined positive torque is a predeterminedvalue that is equal to an impeller stall torque when a turbine speed ofthe torque converter is greater than a minimum impeller stall speed. 8.The vehicle of claim 7, wherein predetermined positive torque is amaximum of the impeller stall torque and a predetermined positive torquevalue.
 9. A method comprising: operating, by a controller, an engine andan electric machine that are coupled to a gearbox through a torqueconverter to cause a torque of the torque converter to achieve apredetermined positive torque at an input of the torque converter inresponse to a driver demand torque at the torque converter decreasing tofall within a range between the predetermined positive torque and apredetermined negative torque.
 10. The method of claim 9 furthercomprising operating, by the controller, the engine and the electricmachine to cause the torque to achieve the driver demand torque inresponse to the driver demand torque being less than the predeterminednegative torque.
 11. The method of claim 9 further comprising operating,by the controller, the engine and the electric machine to cause theelectric machine to achieve the predetermined negative torque inresponse to the driver demand torque increasing to fall within the rangebetween the predetermined positive torque and the predetermined negativetorque.
 12. The method of claim 9 further comprising operating, by thecontroller, the engine and the electric machine to cause the torque toachieve the driver demand torque in response to the driver demand torquebeing greater than the predetermined positive torque.
 13. The method ofclaim 9 further comprising estimating, by the controller, thepredetermined positive torque as an impeller stall torque during a drivecycle based on a model of the torque converter.
 14. A powertrain controlsystem comprising: an engine and an electric machine coupled to agearbox through a torque converter; and a controller programmed to,responsive to a driver demand torque at the torque converter decreasingto fall within a range between an impeller stall torque of the torqueconverter and a predetermined negative torque, command the engine andthe electric machine to cause a torque at the torque converter to be theimpeller stall torque.
 15. The powertrain control system of claim 14,wherein the controller is further programmed to, responsive to thedriver demand torque falling below the predetermined negative torque,command the engine and the electric machine to cause the torque toachieve the driver demand torque.
 16. The powertrain control system ofclaim 14, wherein the controller is further programmed to, responsive tothe driver demand torque increasing to fall between the impeller stalltorque and the predetermined negative torque, command the engine and theelectric machine to cause the torque to achieve the predeterminednegative torque.
 17. The powertrain control system of claim 14, whereinthe controller is further programmed to, responsive to the driver demandtorque increasing to exceed the impeller stall torque, command theengine and the electric machine to cause the torque to achieve thedriver demand torque.
 18. The powertrain control system of claim 14,wherein the controller is further programmed to estimate the impellerstall torque during a drive cycle based on a model of the torqueconverter.
 19. The powertrain control system of claim 14, wherein thecontroller is further programmed to, responsive to a turbine speed ofthe torque converter being greater than a minimum impeller stall speed,limit the impeller stall torque to be greater than a predeterminedpositive torque threshold.